The present invention relates to a howling suppression device and a howling suppression device program as well as an adaptive notch filter and an adaptive notch filter program, and is applicable to an audio receiving device in a speakerphone system for videoconferencing or audio conferencing, for example.
In a speakerphone system for videoconferencing or audio conferencing, etc., sounds (such as audio and voices) output from a speaker is fed back into a microphone, producing acoustic echoes that return back to a transmitting side. Furthermore, if the gain of a feedback system is 1 or more, the system enters an oscillating state (so-called howling). Since howling significantly impedes call, much research and development on howling suppression methods has been conducted heretofore.
As one approach to suppress the howling, there is a method that uses an adaptive notch filter. A notch filter is a filter that removes (attenuates) a specific frequency, while an adaptive notch filter is a filter that adaptively updates filter coefficients to form a notch (rejection frequency). By applying an adaptive notch filter in a howling suppression device and updating the filter coefficient of the adaptive notch filter, it is possible to adaptively suppress a howling frequency. An adaptive notch filter applicable to a howling suppression device is proposed in “A Study on an Algorithm and a Convergence Performance for Adaptive Notch Filter Utilizing an Allpass Filter”, M. Kobayashi, T. Akagawa, Y. Itoh, IEEE Trans., Vol. J82-A, No. 3, pp. 325-332, March 1999 (hereinafter referred to as “Non-Patent Document 1”).
The adaptive notch filter proposed in Non-Patent Document 1 removes a specific frequency by calculating and updating an update amount of the filter coefficient that determines the rejection frequency. The update amount of the filter coefficient to determine the rejection frequency is calculated from an input signal of the adaptive filter and the coefficient in the denominator of a transfer function of the adaptive filter.